In recent years, to meet various applications, image sensors have been improved in performance and diversified in function, and are still in continuous evolution. As one of next generation technologies, photoelectric conversion by organic semiconductor materials instead of by inorganic semiconductor materials can be mentioned. These image sensors are called “organic image sensors.” Further, image sensors (called “stacked imaging devices”) which have spectral sensitivity corresponding to red color, green color and blue color owing to the stacking of plural organic semiconductor layers, are under development, and are attracting interest. These stacked imaging devices do not require a color separation optical system and can extract, from a single pixel, three kinds of electrical signals (image signals) corresponding to red color, green color and blue color, and therefore are high in light availability and large in opening and hardly generate false signals such as moiré. With an image sensor having a conventional color filter, approximately 40% of transmitted light is said to be lost due to absorption by transmission through the color filter.
At present, image sensors that use silicon (Si) as a photoelectric conversion material dominate.
Miniaturization of pixels for improvements in recording density has been advancing, and the pixel size has reached approximately 1 sm. The light absorption coefficient of Si is from 103 to 104 cm−1 or so in the visible light region, and a photoelectric conversion layer in an image sensor is generally located at a depth of 3 μm or greater in a silicon semiconductor substrate. As the miniaturization of the pixel size proceeds, the aspect ratio of the pixel size to the depth of a photoelectric conversion layer increases, consequently leading to leakage of light from adjacent pixels, a restriction to the incident angle of light, and a reduction in the performance of the image sensor. As a solution to such problems, organic materials having a large absorption coefficient are attracting interest. Organic materials have an absorption coefficient of 105 cm−1 or so or greater in the visible light region. In an organic image sensor or stacked imaging device, such an organic material makes it possible to form a photoelectric conversion layer with a reduced thickness, and therefore is considered to realize an improvement in sensitivity and an increase in the number of pixels while preventing false colors. Accordingly, developments of these organic materials are diligently under way.
Organic image sensors are considered to have many advantages as described above, but a capacity reduction of the resulting imaging module can be mentioned as one of problems. The term “imaging module” as used herein means a unit that includes a plurality of built-in organic image sensors and outputs electrical signals, which have been obtained through photoelectric conversion, as an image. Charge obtained under irradiation of light in the photoelectric conversion layer that forms each organic image sensor is converted into a voltage, which is then outputted as an electrical signal (image signal). At this time, if the total electric capacity C is large including, in addition to the electric capacity of the organic image sensor, those of peripheral components such as a floating diffusion (hereinafter abbreviated as “FD”), a buffer amplifier connected to the FD, and a reset gate and horizontal output gate adjacent the FD, the voltage change per charge becomes smaller, resulting in a smaller signal-to-noise (S/N) ratio and deteriorated image quality. Now, the voltage V of an electrical signal is represented by:V=Q/C (Q: quantity of electric charge). Consequently, as the electric capacity of the organic image sensor increases, V decreases, and as a result, electrical signals become weaker. It is to be noted that the electric capacity of the organic image sensor (specifically, the electric capacity of the organic layer to be described next) accounts for approximately a half of the total electric capacity C. Further, the electric capacity C0 is generally represented by:C0=(ε·S0)/d0 (ε: dielectric constant, S0: area, d0: thickness)
Therefore, as factors that affect the capacity reduction of the organic image sensor, the pixel area, the dielectric constants of materials forming the organic image sensor and the thickness of the organic layer in the organic image sensor can be mentioned. If a capacity reduction is attempted depending on the thickness, there is a need to increase the total thickness of the organic layer in the organic image sensor.
As illustrated by way of example in a concept diagram of FIG. 1A, the organic layer has a stacked structure of a carrier blocking layer (first carrier blocking layer) 22, an organic photoelectric conversion layer 23 and a second carrier blocking layer 24, which are all held between a first electrode 21 and a second electrode 25. It is possible to increase the thickness of the organic photoelectric conversion layer 23. However, the organic photoelectric conversion layer 23 is a layer that takes part in the photoelectric conversion function, so that it is often difficult to achieve both thickening and the avoidance of a reduction in photoelectric conversion efficiency upon photoelectric conversion of light of a specific wavelength. Further, if the material forming the organic photoelectric conversion layer 23 in a stacked imaging device has spectral characteristics that also absorb light of wavelengths other than the desired wavelength, the thickening of the organic photoelectric conversion layer 23 has a potential problem of shielding light that the photoelectric conversion layer, which forms the image sensor located in a lower part, is supposed to absorb. The second carrier blocking layer 24 also needs to have a function to transport electrons. If the thickness of the second carrier blocking layer 24 formed of an organic material with low electron mobility is increased, the second carrier blocking layer 24 acts as a resistant component, thereby making it difficult to maintain a high photoelectric conversion efficiency.
Incidentally, the mother skeletons of the following structural formulas (1), (2) and (3) are materials well-known as organic thin-film transistor (organic TFT) materials, and are called “benzothienobenzothiophene (BTBT) materials.” JP 2010-232413A discloses the use BTBT materials in organic TFTs, static induction transistors and solar cells, but makes no mention about their use in image sensors. This patent publication also discloses the use BTBT materials as active layers, and their use in a wide thickness range of 1 nm to 10 μm, preferably from 5 nm to 5 μm, more preferably from 10 nm to 3 μm. In the Examples, the thickness of the BTBT material is set at 250 nm. Moreover, no spectral characteristics are required for the active layers themselves, and no reference is made to electric capacity. WO 2012/121393A1 discloses a technology that liquid crystal is developed using BTBT materials and are used in organic TFTs. However, in this international publication, nothing is mentioned about image sensors, no reference is made to spectral characteristics, and no mention is made about electric capacity.
